Adsorption of an Isocyanurate Silane on E-Glass Fibers from Ethanol and Toluene.

Isocyanurate silane was adsorbed onto E-glass fibers from toluene and ethanol in order to gain a better understanding of the surface processes at the silane-glass interface of composite materials. Use of DRIFT (diffuse reflectance infrared Fourier transform spectroscopy), XPS (X-ray photoelectron spectroscopy), and TGA (thermogravimetric analysis) techniques allowed for the qualitative and quantitative determination of such surface processes. The degree of chemisorption of the silane to the glass surface was investigated by using a sequential washing procedure with solvents characterized by solubility parameters differing from each other. All the analyzing techniques indicated that coupling between the glass and the silane occurred. However, the structure of the silane film depends considerably on the solvent and on the subsequent washing procedure. Copyright 2000 Academic Press.

Layer Buildup on Two-Layered Porous Substrate by Dip-Coating: Modeling and Effect of Additives on Growth Rate.

A model was developed to describe layer buildup on a two-layered substrate during dip-coating. In this model, the growth rate can be used to estimate the permeability of the cast layer. Layers of submicron alumina were dip-coated onto the asymmetric porous substrate to evaluate the model. The layer buildup and properties were studied with different slip additives. Slips were deflocculated with nitric acid or sodium poly(methacrylic acid) (Na-PMAA) to achieve electrostatic or electrosteric stabilization. Properties of the slips with electrostatic stabilization were varied with poly(vinyl alcohol) (PVA) concentration. Properties of the slips with electrosteric stabilization were varied with Na-PMAA concentration. PVA addition decreased the growth rate of the layer by reducing capillary pressure and increasing the viscosity of the filtrate. Electrostatic stabilization of the slip with PVA addition also led to almost complete deflocculation and a rather high density of layers. Na-PMAA addition led to fast growth of the layer. Na-PMAA addition increased the capillary pressure and had a minor effect on the viscosity of the liquid phase. Na-PMAA addition led to an incomplete dispersion of particles and therefore a high porosity of layers. Permeability values of coated layers obtained from the model were compared with permeability values predicted by the Kozeny-Carman equation. In the case of complete deflocculation of the slip, the permeability values estimated with the model correlated reasonably well with the predicted values. Incomplete deflocculation led to higher permeability values than predicted by the Kozeny-Carman equation. Copyright 2000 Academic Press.

Electrosteric Stabilization of Al(2)O(3), ZrO(2), and 3Y-ZrO(2) Suspensions: Effect of Dissociation and Type of Polyelectrolyte.

The mechanisms of eight anionic polyelectrolytes stabilizing colloidal sized alpha-Al(2)O(3), pure ZrO(2), and Y(2)O(3)-doped ZrO(2) particles in aqueous solution are discussed. The polyelectrolytes studied were the Na(+) and NH(4)(+) salts of polyacrylic acid and polymethacrylic acid having different molecular weights. The particle-dispersant interactions were studied by measuring adsorption isotherms, particle size, thickness of adsorbed layer, and zeta potentials by elektrokinetic sonic analysis at different powder volume fractions (straight phi=0.01-0.3), pH, and electrolyte (KCl) content. The dissociation of the polyelectrolytes was studied by potentiometric titrations. The dissociation constant of the polymethacrylates was found to be 0.6 pH unit higher than that for the polyacrylates. High-affinity adsorption isotherms were observed over the pH range when the polyelectrolytes were fully ionized. The results show good correlation between adsorption isotherms and zeta potential data in systems of dispersed, dilute alumina particles. When particles and polymers were of equal charge (the same sign of charge) the polymer shell was thicker. At higher volume fractions (straight phi=0.3), and when alumina particles/added ammonium polyelectrolyte were of equal charge, a maximum in the absolute value of zeta potential resulted. Due to adsorption all the anionic polyelectrolytes studied provided electrosteric stabilization of the alpha-Al(2)O(3), and Y(2)O(3)-doped ZrO(2) suspensions by enhancing the zeta potential to 40 mV or over and by shifting the isoelectric point to lower pH, the low-molecular-weight polyelectrolytes decreasing the isoelectric point more than the polyelectrolytes having higher molecular weight. The polyelectrolytes studied failed to stabilize pure monoclinic ZrO(2) particles. Due to the shortness of the chain of polyelectrolytes studied, no bridging was observed between oppositely charged polyelectrolyte/alumina particles. Copyright 2000 Academic Press.

Specific Adsorption of Nickel and zeta Potential of Silica at Various Solid-to-Liquid Ratios.

Theelectrokinetic curves of silica obtained at given total concentrations of heavy metal cations depend on the solid-to-liquid ratio; namely, at low solid-to-liquid ratios the sign of the zeta potential of silica is reversed to positive at relatively low concentrations of heavy metal cations, while at higher solid-to-liquid ratios the electrokinetic curves are not substantially influenced at much higher concentrations. The surface complexation model fails to properly reflect the Ni effect on the course of electrokinetic curves of silica. The course of electrokinetic curves [positions of the isoelectric point(s) and of the maxima in mobility] is correlated with a linear combination of the surface charge density in the absence of heavy metal cations at given ionic strength and the adsorption density of Ni. Copyright 1999 Academic Press.

Correlation between the Zeta Potential and Rheological Properties of Anatase Dispersions.

The viscosity at an arbitrarily selected shear rate of 1.16 s-1 and the yield stress of anatase dispersions are linearly correlated, and both show a maximum at the isoelectric point (IEP) at pristine conditions. The rheological properties of anatase dispersions respond to the shift in the IEP to higher pH values, which is observed in concentrated solutions of 1-1 electrolytes and in the presence of the organic cosolvents, but the shift in the pH corresponding to a maximum viscosity does not exactly match the shift in the IEP. In contrast with the electrokinetic curves, the rheological curves at high ionic strengths are rather insensitive to the nature of the supporting electrolyte. The acidic branch of the viscosity(pH) curves is relatively insensitive to the ionic strength while the slope of the basic branch decreases when the ionic strength increases. Copyright 1999 Academic Press.

Phase Behavior and Structural Changes in Tetraethylorthosilicate-Derived Gels in the Presence of Polyethylene Glycol, Studied by Rheological Techniques and Visual Observations.

A study has been done on how different concentrations and molecular weights of polyethylene glycol (PEG) affect the phase behavior of the sol-gel reaction. The sol-gel reaction of tetraethylorthosilicate (TEOS) in acidic media in the presence of PEG has been studied by means of visual observations and rheological investigations. The observations revealed that a macrophase separation occurred at intermediate concentrations for the two highest molecular weights of PEG. The proposed mechanism was bridging flocculation between organic and inorganic polymers. At high concentrations of PEG, steric stabilization took place and thus the phase transition evolved without a macrophase separation. Gelation times have been determined by visual observations and two different rheological methods, and there were no significant differences in the gelation times determined by the different methods. Flow curves revealed Newtonian, shear thinning, rheopectic, and thixotropic flow behavior at different stages of the sol-gel reaction, giving information about the structure formed. At the gelation point a viscoelastic scaling law G'(omega) approximately G"(omega) approximately omegan has been observed with values for the critical exponent n in the range of 0.71-0.83. Values of the critical exponent have been used for gaining information about the degree of branching of the gel when comparing different phase behavior and growth mechanisms. It was shown that the phase-separated gels were less branched and more open than the others. Copyright 1998 Academic Press.

Evaluation of the Structure and Acid-Base Properties of Bulk Wood by FT-Raman Spectroscopy.

The structure of pine wood (Pinus silvestris L.) has been analyzed by FT-Raman spectroscopy, taking birch wood and the wood components cellulose, hemicellulose (xylan), and lignin as well as previously characterized wood resins as references. The acid-base properties of bulk pine wood were evaluated by comparing the spectra recorded before and after the treatment with various solvents. After the treatment with the probe liquids having only a Lifshitz-van der Waals (LW) component, it was found that the LW interactions in pine wood take place without changing the main structure. After treatment with Lewis acid-base active probe liquids, the spectra indicate that, e.g., the intense peak located at approximately 2936 cm-1 (CH2 stretch) seems to disappear, suggesting that this peak may be related to Lewis acidity. In addition, after treatment with a Lewis acid, it was found that the intense peak located at approximately 1657 cm-1 (C&dbond;C) is shifted, relating to Lewis basicity. With the ratio approximately 2936/ approximately 1657 cm-1 as a measure of the acid-base properties of bulk wood, a value of about 2.00 indicates that the bulk pine wood is largely acidic. The pH determined supports the evaluation made by FT-Raman spectroscopy. Copyright 1998 Academic Press.

LETTER TO THE EDITOR

An isocyanurate silane coupling agent effectively attaches to E-glass fibers both from aqueous or CCl4 solution. SEM revealed remarkably different glass surface topographies that were solvent dependent. Application of the coupling agent from CCl4 solution gave rise to unprecendented polymeric bridges between adjacent parallel fibers. The existence of such large bridging structures has only recently been postulated, and it was not thought that such theoretical projections would ever be observed, as polymeric bridging structures typically should collapse in the absence of solvent. Copyright 1997 Academic Press. Copyright 1997Academic Press

Treated Glass Fibers-Adsorption of an Isocyanurate Silane from CCl4.

The adsorption of isocyanurate silane on E-glass fibers has been investigated by scanning electron microscopy, diffuse reflectance Fourier transform infrared spectroscopy (DRIFT), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The strength of the silane-glass interaction has been related to the extent of removal of the adsorbed species after washing with a range of solvents. The DRIFT and TGA methods both support the conclusion that tetrahydrofuran and acetone, used as washing liquids, adsorb onto the polymerized silane surface. Dichloromethane efficiently removes most of the physisorbed silane, while methanol is less efficient, but promotes a reorientation of the silane. The adsorption and the orientation of the silane is clearly dependent on the relative affinity for the solvent on the one hand and for the glass fiber surface on the other. The XPS N/O ratio and N 1s signal defined both the silane removal and its reorientation on the fiber surface. The morphology of the treated fibers was studied by scanning electron microscopy. Copyright 1997 Academic Press.